Today, we walk in some very old footsteps. The University of Houston's College of Engineering
presents this series about the machines that make our civilization
run, and the people whose ingenuity created them.

A million and a half years ago, a band of our Homo erectus forebears
walked along the shallow shore of a river in Ileret, Kenya. It was after a seasonal flood and
their footprints were preserved in the claylike mud. Twenty-thousand years later, another group
left another layer of prints there. They too were covered over and preserved. Now archaeologists
have found them.

These are the second oldest known hominid tracks. The other set dates all the way back to
Lucy. The 3.6 million-year-old Laetoli prints in Tanzania show Lucy, or her Australopithecus
kin, walking on two legs. Now we have two snapshots of our evolution into bipedal beings.
(You might wonder how we know their dates so well. The answer: Western Africa was frequently
sprinkled with volcanic ash. It contains minerals that decay radioactively at a highly
predictable rate. So geologists can calculate the age of these old footprints within a
fraction of a percent.)

Lucy's Australopithecus feet aren't well defined, but it is clear that she'd made the
transition from ape-walking to human-walking. An ape rolls forward, heel to toe on the
outside of its foot, while its long thumb-like big toes splay in toward each other. A
human continues that rolling motion across the front of its foot, to a much shorter,
but load-bearing, big toe. By the way, the much older Lucy tracks may be less clear,
but we can tell that a male and female walked side by side, close enough that they must
be touching. I like to imagine them holding hands.

Unfortunately, the bones of Lucy's feet were never found. The Laetoli prints are all we
have. So two questions linger: How many of the 26 bones of a modern human foot did she
have? And exactly how did she walk upon those bones?

We don't have Homo erectus' foot bones either. But the Ileret footprints remain clear
enough after a million and a half years to tell us that we walked then, much as we do now.
The varying depth of the footprints reveals how weight was distributed. It's consistent
with a full roll from heel, up the outside, and across the toes. Our African grandparent
executed the same delicate ballet that we do as we stroll along sidewalks -- or along
riverbanks.

More emerges from all this, of course. The natural selection of physical traits gave
arboreal apes that long toe, so useless on the ground. They can grasp tree limbs while
you and I, and Homo erectus, remain on the ground. But Homo erectus could chase down
prey, while his hands were free to hurl a rock -- or a spear.

Lucy and her companions lived almost two million years earlier still. And they could
already walk, hand in hand, by the riverside -- even if they were still working on that
wonderfully complex foot roll. Now the Ileret site tells us that Homo erectus, whose
brain was maybe two-thirds the size of yours and mine, had developed feet that could
run with the best of us.

I'm John Lienhard at the University of Houston,
where we're interested in the way inventive minds
work.

(Theme music)

R. H. Crompton and T. C. Pataky, Stepping Out. Science, Vol. 323, 27
Feb. 2009, pp. 1174-1176. See also the full technical article attributed to
12 authors in the same issue: Early Hominin Foot Morphology Based on
1.5-Million-Year-Old Footprints from Ileret, Kenya. pp. 1197-1201. For more
on these articles,Click Here.

For more on the Laetoli site, see: Laetoli: A Pliocene Site in Northern
Tanzania. M. D. Leakey and J. M. Harris eds., (Oxford: Clarendon Press, 1987)

For a general account of early human evolution with more on footprints, see C. J.
Cela-Conde and F. J. Ayala, Human Evolution: Trails from the Past.
(Oxford: Oxford University Press, 2007).